Sequential-simultaneous colour television system using a frequency modulated subcarrier



Oct. 7, 1969 G. MELCHIOR 3,471,635

SEQUENTIAL-SIMULTANEOUS COLOUR TELEVISION SYSTEM USING A FREQUENCYMODULATED SUBCARRIER Filed July 26, 1966 2 Sheets-Sheet 1 I Fig FC=42Q0FO=FC+ 116 I I I I l 3500 $700 $00 4100 F0 Fig.1

lNVEA/TO 1e: G RHRD HACl-HOR Kmmm Agent United States PatentSEQUENTIAL-SIMULTANEOUS COLOUR TELE- VISION SYSTEM USING A FREQUENCYMODU- LATED SUBCARRIER Gerard Melchior, Asnieres, France, assignor toCompagnie Fraucaise de Television, a corporation of France Filed July26, 1966, Ser. No. 567,986 Claims priority, applica6tig37France, July30, 1965,

Int. Cl. H04n i/46, 9/02, /38

US. Cl. 178-5.2 5 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates to improvements to the sequential-simultaneous colourtelevision system with memory, using a frequency-modulated subcarrierfor the alternate transmission of two chrominance signals, such as theSECAM system. More particularly, it relates to protecting the colourinformation against noise.

In a system of the SECAM-type using a frequency modulated subcarrier,before its addition to the luminance signal the subcarrier is filteredin a so called coding filter having a maximum attenuation at frequency Flocated near the center of the subcarrier frequency band, and Whose gainincreases on both sides of this frequency. In systems according to priorart, this frequency F, coincides with the resting or center frequency ofthe subcarrier corresponding to a zero amplitude modulating(chrominance) signal.

This filtering by the coding filter, also referred to as high-frequencypre-emphasis, provides on the one hand improved campatibility of thissystem for black-and-white (monochromatic) reception and, on the otherhand, in conjunction with the so called decoding filter, whosecharacteristic is the inverse of that of the coding filter, providesincreased protection of the subcarrier against noise.

The noise reduction obtained by using the coding and decoding filtersfor the transmission of a frequency modulated signal is not uniformthroughout the frequency band occupied by this signal. As some colours(for instance, red) are more sensitive to noise than others, the objectof the present invention is to provide better overall performance bygiving the colours most vulnerable to noise interference an increasedprotection with respect to other colours less sensitive to noise. In theSECAM system using a frequency modulated subcarrier, each colour isrepresented by a particular frequency shift relative to the center(resting) frequency. It is therefore possible to obtain better noiseprotection for certain colours by shifting the resting frequency withrespect to F which also means the shifting of the frequency representinga particular colour relative to P For example, experience has shown,that it is possible to obtain a better protection against noise for thered colour by positioning the resting frequency of the subcarrier awayfrom F toward the frequency correspond- 3,471,635 Patented Oct. 7, 1969ice ing to the maximum algebraical value of the difference signal (RY).

In the preferred embodiment of the SECAM system, the alternatelytransmitted colour signals, A and A are respectively of the form (RY) /Kand (BY)/K where RY and B--Y are the well known difference signalsobtained from the basic colours red R, blue B, green G, and theluminance signal Y, and K and K two constants with opposite signs. It istherefore possible to obtain increased noise protection for both the redand the blue colours by using two different resting frequencies, F and Frespectively, located on both sides of F for the respective transmissionof the two signals A and A According to the invention, there is provideda colour television system comprising a transmitter and at least onereceiver, in which the composite video signal comprises a luminancesignal and a colour subcarrier. The transmitter includes a colourchannel comprising means for generating two colour signals A and A meansfor pre-emphasizing said two colour signals, and means for alternatelyfrequency modulating the subcarrier by the two pre-emphasized signals Aand A where the alternating occurs at the line frequency. The modulatedsubcarrier is filtered in a coding filter Whose gain increases on eitherside of a frequency F through the variation range of the instantaneousfrequency of the frequency-modulated subcarrier. The receiver includinga colour channel fed by the modulated subcarrier comprises a decodingfilter which compensates for the frequency selective action of thecoding filter. The modulated subcarrier is fed to a direct channel and adelay channel in parallel, where the delay channel holds the signalpropagating therethrough by one line period. The receiver also includesswitching means having a first and a second output and whose inputs arerespectively fed by the direct and the delay channels, said switchingmeans directing the subcarrier to the first or to the second outputaccording -to whether it is modulated by A or by A Two frequencydiscriminators respectively receive the outputs of the switching means.To obtain better noise protection for colours particularly vulnerable tonoise interference, the transmitter further comprises means for fixingthe resting frequency of said subcarrier for the transmission of signalA to a value F and for the transmission of signal A to a value F whereboth frequencies F and F5 differ from the frequency F Each of thediscriminators of the receiver are also respectively centered on thefrequencies F and F,,.

It will be recalled here that in the prior art the resting frequency ofthe subcarrier is its instantaneous frequency for the zero value of thepre-emphasized signal A (z'=l or 2), and that the correct rendition ofachromatic (i.e. white or gray) parts of the transmitted picture, whichis particularly important for the viewer practically requires that thefrequency demodulaters should be centered on the resting frequency, astheir zero values are more easily precisely adjusted than any othercritical value.

However, the advantages accruing from the use of two different restingfrequencies for A and A are not obtained at the expense of morecomplicated receivers since the usual embodiment of the receivers of thesystem to which the invention is applied already comprises two differentdemodulators for supplying respectively signals A and A The inventionwill be further explained, and other features will become apparent, fromthe following description, with reference to the accompanying drawings,in which:

FIG. 1 shows the relative position of frequency F and of the two restingfrequencies of the subcarrier according to one embodiment of theinvention;

FIG. 2 is the diagram of a preferred circuit for modulating thesubcarrier in a transmitter according to the invention;

FIG. 3 shows a modification of the diagram in FIG.

FIG. 4 is a diagram of an embodiment of the subcarrier channel in areceiver according to the invention.

There will now be described in a detailed manner, by way of anon-limiting example, the improvement according to the invention asapplied to a transmitter-receiver system defined as follows:

The subcarrier channel extends from 3.5 to 4.8 mc./s.

The chrominance signals are A =(RY) /K and A =(BY)/K The luminancesignal Y has the form R, B and G are the primary colour signals,gammacorrected, varying between and 1.

The maximum algebraical value of RY occurs when R=1, B=0, and 6:0.

RY and B-Y vary therefore, theoretically, between 0.7 and +0.7 andbetween 0.89 and +0.89, respectively. However, in practice, thesesignals do not exceed of their theoretical maximum absolute values, andvary therefore between 0.525 and +0.525, and between 0.67 and +0.67respectively.

Accordingly, the absolute values of K andK are taken equal to 0.525 and0.67 respectively, which gives the same range of variation to the twosignals A and A More precisely, the signals A and A obtained at theoutputs of a matrix receiving at its three inputs the three gammacorrected primary colour signals, are subjected to a filtering inreducing their bandwidth to 1.5 mc./s. for example, and are then appliedto two pre-emphasis filters whose gain increases with frequency.

This pre-emphasis, as is well known, causes substantial amplitude peaks(both positive and negative ones) in steep transitions of the videosignals, Consequently, the pre-emphasized signals are subjected to bothpeak and base clipping in slicers. The clipping is kept sufiicientlymoderate to prevent introduction of substantial distortion of thesignals restored in the receivers, since this operation, contrary to thepre-emphasis, is not compensated for in the receivers.

The output signals from the slicers are applied to the first inputs oftwo adders whose second inputs respectively receive during two so-calledidentification signals a =pa and a =qa, signal a having a singlepolarity and p and q being two constants of opposite signs. As signal a,each of the two signals a and a like signal a, is repeated from one lineto the next within each checking period which occurs during a part ofeach vertical blanking interval. These identification or checkingsignals are used to determine the correct phase of the switch in aSECAM-tpye receiver and their absence is used to activate the colourkiller in such a receiver as was described in detail in prior U.S.Patent No. 3,267,208, Color Identification and Associated Apparatus inSequential Color Television Systems patented Aug. 16, 1966, by D.Brouard.

The outputs of these adders are connected to the two signal inputs of anelectronic switch with a single output, changing its state during eachhorizontal blanking interval under the influence of a signal applied toits control input.

The switch supplies therefore at its output alternately the signals Aand A during the active field periods, and alternately the signals a anda during the checking periods.

This output signal of the switch is used for frequentlyrnodulating thesubcarrier.

In order to minimize the visibility of spurious patterns due to thepresence of the subcarrier in the upper portion of the spectrum of theluminance signal, the frequency '4 modulation is effected in such a waythat the subcarrier has the same phase at the beginning of each activeline duration. The subcarrier is subsequently submitted to a series ofdiscrete phase-shifts. For example, at least while picture signals arebeing transmitted, its phase is shifted by degrees for the duration ofone line out of three successive ones, and moreover by a further 180degrees for the duration of one field out of two successive ones.

These phase-shifts i.e., 180 degrees for alternate fields in addition to180 degrees every third line, with a 625 line picture and interlacedscanning, insure at the beginning of lines of the picture in thevicinity of one another, and corresponding to the transmission of thesame colour signal A, (i=1 or 2), phase opposition of the subcarrier,leading to a satisfactory reduction of visibility of the spuriouspatterns.

The above-mentioned system uses in the transmitter, as part of a devicefor protecting the chrominance signals against noise, a so-called codingfilter, whose input is the modulated subcarrier is applied and whosegain characteristic rises steeply on either side of a predeterminedfrequency P of the instantaneous frequency variation range of thesubcarrier. The frequency F in the known art, is equal to the (single)resting frequency of the subcarrier.

The subcarrier is thereafter added to the luminance and the sync signalsto form the composite video signal which is then transmitted by thetransmitter.

In the receiver, the modulated subcarrier, filtered from the compositevideo signal, is applied to a so called decoding filter compensating forthe phase and amplitude distortion imparted by the coding filter.Consequently, the decoding filter has a gain characteristic inverse ofthat of the coding filter, with a maximum gain for frequency F Thesignals A and A are repeated to be made simultaneous and switched to twodifferent channels.

Preferably, the repetition and switching occur at subcarrier frequency,and signals A and A are then obtained by means of two frequencydemodulators, and de-emphasized afterwards.

As has been shown in the applicants prior U.S. Patent No. 3,365,541, thecombining of pre-emphasis-de-emphasis with coding-decoding provides thefollowing results:

(a) The pre-emphasis ratio remains sufficiently moderate to avoidexcessive increase of subcarrier bandwidth;

(b) The presence of the subcarrier in the luminance signal does notperceptibly deteriorate the picture quality, particularly inblack-and-white television receivers; and

(c) The protection of chrominance information against noise is generallysatisfactory.

The applicant has found that, in a general way, it was advantageous toposition the minimum of the characteristic of the coding filter, andconsequently the maximum of the characteristic of the decoding filteraway from the resting frequency of the subcarrier, towards theinstantaneous frequency corresponding, to the chrominance signal havingthe higher absolute value for the maximum value of RY the preemphasispeaks which do not occur in zones of uniform colour are heredisregarded.

With the two chrominance signals A and A more particularly considered inthis example, i.e. A =(RY)/K and A =(BY)/K this means that the shift ofF should be toward the lower or higher frequencies according to whetherK is negative or positive. The slope of the frequency-modulationcharacteristic (frequency vs. voltage) being here and hereafter assumedto be positive.

The advantage of this step may be explained as follows:

The chrominance channel noise suppressing process is made up of thepreemphasis-coding processes in the transmitter, and thedecoding-deemphasis processes in the receiver as explained in Patent No.3,365,541. By coding and decoding are meant the passing of the modulatedsubcarrier through the coding and the decoding filters respectively.

As concerns the useful signal, the deemphasis compensates for thepreemphasis, and the decoding compensates for the coding.

However, one obtains a reduction of the noise introduced during thetransmission, due to the two windows formed in the noise spectrum by thedecoding filter and by the deemphasis filter.

However, the protection against noise is not uniform for the differentcolours.

Actually, disregarding the action of the filters, the noise energydensity is statistically uniform at subcarrier frequency.

However, as results from a well-known property of frequency modulation,this uniform noise energy spectrum is transformed into a parabolicdistribution of noise energy in the demodulated signal, wherein thenoise energy density rises proportionally to the square of frequency.

The deemphasis filter, whose gain decreases with increasing frequency,counteracts this increase of noise at higher video frequencies.Nevertheless, in the application under review, this compensation cannotbe achieved in as large a measure as would be desirable, as this wouldrequire an excessively high preemphasis rate and in consequence aninadmissible widening of the bandwidth of the subcarrier channel.

But, to the action of the deemphasis filter is added that of thedecoding filter which reduces the noise at the subcarrier frequency andthus, indirectly, at video frequency.

Theoretical considerations show that, as concerns the reduction of noiseat video-frequency, the action of a decoding filter, with acharacteristic G(F), of the power gain G as a function of the frequencyF, may be practically assimilated, for an instantaneous frequency F ofthe subcarrier, to that of a video frequency filter with thecharacteristic g for the power gain g, as a function of the frequency f,with The action of the decoding filter is thus dependent on theinstantaneous frequency, consequently on the instantaneous value of thetransmitted chrominance signals, and finally on the colour to bereproduced. By positioning the maximum of the characteristic of thedecoding filter and consequently the minimum of the characteristic ofthe coding filter as indicated previously, it is possible to providebetter noise protection for certain colours, which are chosen from thosemost vulnerable to noise interference, at the price of accepting lesserprotection for other colours, which are less sensitive to noise.

In a general way, the action of the decoding filter remains satisfactoryas long as the interval between the filter center frequency E and theinstantaneous frequency F is not too wide. The above-mentionedpositioning of F away from the resting frequency towards theinstantaneous frequency corresponding to the transmission of the maximumalgebraical value of RY, allows a big improvement of the protectionagainst noise of highly saturated red portions of the picture, wherenoise is particularly disturbing to the observer, for different reasonseither objective or subjective.

The precise position of E, with respect to the resting frequency to beadopted, taking into account the distribution of noise at videofrequency, the action of the deemphasis filter, and the effects of theshift for other colours, may be adjusted experimentally to obtain asatisfactory overall result.

The applicant has found in particular that if K is chosen with the samesign as K not only the red, but also the other two colours of the 75percent standard bar pattern, which have the lowest brightness and forwhich the eye of the viewer is therefore particularly sensitive to noiseare favoured by the above-mentioned measure.

However, by taking for K and K the same sign, the visibility of errorsin the chrominance signals resulting from the addition of the subcarrierto the luminance signal is increased.

Assuming, in fact, that the luminance signal Y is repeated at leastapproximately, which is generally the case, along two picture linesanalyzed successively, the errors due to this cross-colour, andespecially to the differential phase, will be substantially the same forthe signals A and A relating to these two picture lines.

If K and K have the same sign, the values of RY and BY will be affectedin the same direction, whilst, if K and K have opposite signs, thevalues of RY and B-Y are affected in opposite directions.

It can now be shown, by using the colour triangle and taking intoconsideration the sensitivity of the human eye to chromaticitydifferences, that the eye will generally be more sensitive tochromaticity errors if the values of R-Y and BY are affected by errorsin the same direction than if they are affected by errors of oppositesigns.

An advantage of the present invention is that it allows the transmissionof R-Y and BY with coefficients having opposite signs while maintainingin a large measure the advantages accruing from positioning F away fromthe resting frequency simultaneously towards the instantaneous frequencycorresponding to the transmission of the maximum algebraical value ofR-Y for the transmission of signal A and towards the instantaneousfrequency corresponding to the transmission of the maximum algebraicalvalue of BY for the transmission of signal A This is accomplished bykeeping the E fixed and shifting the resting frequency of the subcarrierfor RY and for B- Y.

Taking into account other considerations which will be set forthhereinafter, a satisfactory embodiment of the present invention for thesystem considered corresponds to the following data: K is negative, andequal to 0.525"; and K is positive, and equal to +0.67.

The preemphasis of signals A and A is effected according to the law:

1+ (f lfr) 1+ f/kf1 with f kc./s., for f in kc./s., k=3; g (f) being thepower gain of the preemphasis filter for frequency f.

The power gain G of the coding filter for frequency These two gains g(f) and G (F), are power gains in the strict sense of the term, i.e.true ratios and not db.

Experience has shown that optimum overall performance is obtained bychoosing F :44 mc./s. and F' =4.25 mc./s., approximately. As theprocedure for reducing the visibility of the subcarrier in the SECAMsystem requires resting frequencies which are entire multiples of theline frequency F the resting frequencies of the subcarrier in thepreferred embodiment of this system are therefore:

This gives, for a 625 line picture and 50* fields per second, i.e. F=15.6-25 kc./s. the following values: F,,=4.406 25 mc./s., and F",=4.250mc./s.

The restricted frequency swing will be defined as the range of variationof the instantaneous frequency corresponding to the range of variationfrom -l to +1 volt of the preemphasized signal A, (i=1 or 2). Thiscorresponds to the positive maximum and negative minimum of theunpre-emphasized signal, i.e., of the chrominance signal withouttransients. It will be remarked that this range of variation is the sameas that of the signal A not preemphasized.

The restricted frequency swings were chosen to be: from F 280 kc./s. toF +280 kc./ s. for signal A from F',,230 kc./s. to F',,+230 kc./s. forsignal A as will be explained hereinafter.

The total" frequency swing, i.e. the entire range of variation of theinstantaneous frequency, corresponding to the extreme values of theclipped pre-emphasis peaks, is substantially the same for the twosignals and extends: from F =3.90O mc./s. to F =4.756 mc./s.

The excess of the total frequency swing over the restricted frequencyswing is only used as concerns A and A for the transmission of highpre-emphasis peaks and the clipping of each of the two signals A and Ais effected accordingly. F is taken equal to 4.290 kc./s.

Of course, this is only a numerical example.

However there will be indicated, to show how the invention may beapplied in a general way, the reasons leading to those dissymetricaldata.

As will be explained hereinafter, it has been found sufficient to use,for signal A an upper sideband of about 400 kc./ s. only, as comparedwith 900 kc./s. for the lower sideband.

It will be first recalled that in frequency modulation, the transmittedspectrum of the modulated wave comprises the frequency swing (variationinterval of the instantaneous frequency) bordered generally bytwomarginal bands, forming part, one of the upper sideband and the otherof the lower sideband. The marginal bands considered here are thosewhich border the restricted frequency swing.

Thus, the reduction of the upper sideband to 400 kc./s.

for the signal A takes the following factors into consideration:

- (1) From the point of view of compatibility, it is of greater interestto reduce the upper sideband than the lower sideband as this allows forthe shifting of the resting frequency toward the higher frequencies andas most of the subcarrier spectral energy is concentrated around theresting frequency, the subcarrier will interfere less with the luminancethe higher its frequency.

(2) A sufficiently correct definition of the transmitted signal can bemaintained with one of the marginal bands being considerably reducedprovided that the other one is sufficiently wide.

(3) In order to ensure a subjectively satisfactory transmission of RY,it is more important to respect the preemphasis peaks for the values ofA corresponding to a positive value of R-Y rather than for the values ofA corresponding to the negative values of RY. This can easily be seenfrom the fact that positive pre-emphasis peaks mean abrupt transientstoward higher brightness in general, which are much more perceptiblethan transients toward lower brightness, i.e., darker areas in thepicture, which are represented by negative-going pre-emphasis peaks.

As the upper limit of the subcarrier channel is approximately at 4.8mc./s., the 400 kc./s. width of the upper sideband sets the restingfrequency 1 of the subcarrier at about 4.4 mc./s. insofar as the signalA is concerned.

Regarding A and for reasons outlined above, the sign of K is opposite tothat of K i.e. K is positive and equal to +0.67. Taking into accountthis positive sign, it is preferable to make the upper sideband of themodulated subcarrier wider than that of the upper sideband of thesubcarrier signal modulated by A without, however, it being necessary tomake it as wide as that of the lower sideband used for A taking intoconsideration, on the one hand the special importance of the saturatedred zones, as already mentioned hereinbefore, and on the other hand thefact that, from the viewpoint of definition, it matters little which ofthe two marginal bands is reduced relative to the other. One may have,for example, 550 kc./s. for the upper sideband relating to A whichmeans, for the resting frequency F a value of the order of 4.250 mc./s.

The restricted frequency swing may have, for A the same width as for AIt is however preferable to reduce it, for example, to F' i230 kc./s.,since the protection of the signal A against noise, still remainssatisfactory and this reduction has the additional advantage ofcompensating in part for the decrease in bandwidth of the wider marginalband retained for A compared to the one retained for A this decreasebeing due to the positioning of F with respect to F It will be seen thatthe description as presented, as hereinabove, may be viewed as the useof two different slopes for the modulation characteristics respectivelyused for the transmission of signals A and A having the same range ofvariation, or as the use of two constants K and K for signals A and Asuch that the ratio the two signals A and A having then differentvariation ranges, but the same slope being maintained for bothmodulation characteristics.

The minimum of the gain characteristic of the coding filter is locatedbetween F and F,,, but taking into account the better overall protectionto be given to A comparatively to A the difference of the restingfrequencies F and F with respect to F are not equal, and the minimum islocated at a frequency of the order of 4.290 mc./s., i.e. a differenceof only 40 kc./s. with respect to F',,, against a difference of about116 kc./s. with respect to F FIG. 1 shows the centre part of the gainvs. frequency curve of the coding filter, translated into decibels, withindication of the limits of the chrominance channel and of the locationof the frequencies F and F on the frequency axis. Naturally, the gaincurve of the decoding filter is the reverse of that of the codingfilter.

FIG. 2 illustrates a particularly advantageous embodiment of thesubcarrier generating circuit, which has the advantage of insuring witha single frequency modulated oscillator, two stable resting frequenciesF and F and of allowing the use of the same system for subcarriervisibility reduction, which is used in known art with a singlesubcarrier resting frequency.

The drawing shows the switch 24 having two inputs and one output, andsupplying the signal to be transmitted by modulation of the subcarrier.

The two inputs of the switch are connected to the outputs of twocircuits 101 and 102 respectively. The former supplies signal A duringeach active field duration i.e. during each time interval comprisedbetween two successive vertical blanking intervals, and signal a, duringthe checking periods. The latter supplies signal A during each activefield duration and signal 11 during the checking periods. In thisembodiment, signals A and A supplied by circuits 101 and 102 arepreemphasized and clipped before the switching the ratio of theconstants K and K included in the expression of A and A respectively aretaken such that the same slope of the frequency vs. voltage curve of themodulator may be used. Identification signals a and a are preferablyadded to chrominance signals A and A respectively, after the latter havebeen preemphasized and clipped. Thus the indentification signal a and awhich respectively have the form of trapezoidal pulses of oppositepolarities, are added to the preemphasized and clipped chrominancesignal during check- 9 ing periods included in each vertical blankinginterval, as described in Patent No. 3,267,208.

The switch 24 changes its state during each horizontal blankinginterval, due to the control signal applied to its control input 30.

The output signal of switch 24 presents during each horizontal blankinginterval a constant value, the socalled blanking level. This circuit 24feeds the first input of an adder 4, whose output is connected to thesignal input 12 of the clamping circuit 3.

The clamping circuit 3 is a conventional clamping circuit of a typeoften used in television as a D-C restorer for video signals. Itincludes a signal input for receiving the video signal, a control inputfor receiving clamping pulses whose duration coincides approximatelywith that of the horizontal blanking intervals, and a reference inputbrought to the reference potential to which the blanking level is to beclamped. These three inputs are, in FIG. 2, inputs 12, 13 and 14respectively. The control input 13 of the clamping circuit is connectedto a general input 2 of the circuit to which the conventional SECAM-typeclamping pulses are applied.

The output of the clamping circuit 3 is connected to the modulationinput of a frequency modulated oscillator 7 of the general type in whichthe instantaneous frequency is determined by the value of the signalapplied to its modulation input.

The output signal of the oscillator 7 is applied, through a limiter 6 ifnecessary (i.e. if the oscillator 7 supplies a frequency modulated waveaffected by a parasitic amplitude modulation), to the first input of aphase comparator 8, whose second input is connected to the output of aswitch 39, having two signal inputs connected, respectively, to theoutputs of two harmonic generators 19 and 29, supplying two referencewaves. The switch 39 has a control input 40, receiving the same controlsignal asthe input 30 of the switch 24.

Each of the two harmonic generators 19 and 29 comprises in series twoamplifiers, the load of each amplifier being a crystal resonant circuithaving a very high selectively (i.e. Q). The resonant circuits of thegenerator 19 are tuned to the frequency F =282-F and those of thegenerator 29 to the frequency F',,=272-F The two generators areshock-excited by pulses I obtained by differentiation of the leadingedge of the clamping pulses, and these pulses J are applied to the input43, connected to the inputs of the generators 19 and 29.

The two generators are therefore synchronized in phase at the start .ofeach clamping pulse, and the phase shift between their two outputsignals 1 s. later, attains only 360 -10'F 1O which is less than 60.

The phase comparator 8 supplies, for example a signal in sin P, where Pis the phase difference between its two input signals.

The output of the comparator 8 is connected through an amplifierarrangement 5, which may be of the lowpass type, to the reference input14 of 'the clamping circuit 3. This amplifier amplifies the signalsupplied by the phase comparator 8 and adds to the amplified signal a DCvoltage N The oscillator 7 feeds, in addition to the limiter 6, anotherlimiter 81, whose output is connected to subsequent subcarrier circuitsincluding the circuits 82 imparting the above mentioned 180 phase-shiftsand the coding filter 83.

The circuit comprises finally a pulse generator 1, whose input isconnected to the input 2, and whose output is connected to the secondinput of the adder 4.

The operation of this circuit may be explained as follows:

First, it will be assumed that switch 39 is held permanently in itsfirst state, where it delivers the output signal of the harmonicgenerator 19. Further, it will be assumed that pulse generator 1 andadder 4 are disconnected from the circuit, the output of switch 24 beingdirectly connected to the signal input of the clamping circuit 3.

The clamping circuit 3 operates as a conventional clamping circuit,except for the nature of the reference potential. There is thusobtained, for the duration of each clamping pulse, a loop 7-6853-7synchronizing in frequency and phase the wave supplied by the oscillator7 :and that which is supplied by the harmonic generator 19, since theequilibrium cannot be reached unless they have the same frequency and aconstant phase difference go between them.

It follows that: (a) because of the lasting effect, during the followingactive line duration, of the clamping effected by circuit 3, the restingfrequency of oscillator 7, is during this line active duration,effectively set at the frequency P of harmonic generator 19, and anydrift of oscillator 7 is corrected; (b) at the start of each active lineduration, the wave supplied by oscillator 7 has a fixed phase 5 If nowswitch 39 is maintained in its second state, the process will be thesame as that which has been described above, except that the restingfrequency of the oscillator 7 will be set at F and that the phase of thewave supplied at the beginning of each active line duration by thisoscillator will have a fixed value, generally different from dependingupon the phase difference (p' between the phase of the oscillator 7 andthat of the generator 29 when equilbrium is reached in the feedbackloop.

If switch 39 changes its state in synchronism with switch 24, thedesired result will be achieved as concerns the resting frequencies.Besides, the phase of the subcarrier will be the same at the start ofeach active line duration corresponding to the transmission of A andwill have also a constant value at the start of each active lineduration corresponding to the transmission of A The D-C component Nadded to the amplifier error signal in the amplifier 5 is chosen so asto facilitate the synchronizing. It may, for example, be takenintermediate between the control voltages for the oscillator 7corresponding nominally to frequencies F and F, of the latter.

The elements 1 and 4 of this circuit have the object of facilitatingthis control, because the equilibrium might not be reached during theclamping periods if the initial phase difference P at the start of theclosing period of the loop differs too much from that corresponding tothe equilibrium state. Especially, if for reasons of accuracy, a phasecomparator is used which supplies an output signal of the form sin P,the duration for establishing the equilibrium state becomes very long ifthe initial phase difference between the wave supplied by oscillator 7and the reference Wave differs by about 180 from the phase differencewhich corresponds to the equilibrium state. This shows the utility offacilitating the control by acting quickly by other means on the phaseof the modulated oscillator so that a favourable phase difference, i.e.not too far from the one which the feedback loop tends to impart isavailable well before the end of the clamping pulses.

The pulse generator 1 is used for this rapid action. It receives theclamping pulses and supplies blocking pulses.

The generator 1 comprises for example a circuit, differentiating theleading edges of the clamping pulses, which thus delivers a short pulseat the start of each clamping pulse, followed by a blocking oscillatortriggered by these short pulses and supplying for each of them a pulsein the form of a trapezium with steep flanks covering only the beginning(about 1 ,uS.) of the duration of the clamping pulses.

This pulse is applied to the second input of the adder 4, so that duringthe corresponding time interval the output signal thereof is the sum ofthe blanking level and of the blocking pulse.

To each trapezoidal pulse corresponds a blocking, followed by anunblocking of the oscillator 7 which will start to oscillate again andhave a substantially constant phase at an instant t, separated from thetrailing edge of the trapezoidal pulse by a fixed time interval T longenough for the transient state of oscillator 7 i.e. the state before theoscillation becomes subtantially stable, to have come to an end.

The unblocking of the oscillator 7 causes the closing of the feedbackloop.

Experience shows that the gain of the feedback loop and thecharacteristics defining the blocking pulse may be adjusted to insureequilibrium of the feedback loop before the end of the clamping pulses,whether the output signal of generator 19 or of generator 29 is used asreference.

This condition may be obtained in both cases since, on the one hand, asindicated above, the phase difference between the two reference waves isless than 60 1 ,us. after the beginning of each horizontal blankinginterval, and, on the other hand, the diflFerence between and qo' may bemade very small through an adequate gain of the feedback loop.

This device may be further improved by the modification of FIG. 3.

The blocking pulses are directly produced by a blocking oscillator 11triggered by pulses corresponding to the leading edge of the clampingpulses previously applied to input 2, one output of the generator 11being connected as previously generator 1 to one input of the adder 4.On the other hand fresh clamping pulses are obtained from a blockingoscillator 50 triggered by the trailing edge of the blocking pulseswhich are to this end differentiated in a dilferentiator 52 whose inputis connected to the output or the oscillator 11 and whose outuput isconnected to the input of the oscillator 50. The pulses I applied togenerators 19 and 29 correspond to the leading edge of the freshclamping pulses.

The output of the oscillator 50 is connected to the first input of theclamping circuit 3.

This arrangement makes it possible to separate the synchronizing andblocking operations better.

Another method for blocking oscillator 7 consists in inserting into thecircuits of oscillator 7 a switch, actuated by the blocking pulses, sothat for the duration of these pulses, one or more elements, essentialto the generation of oscillation, be disconnected or short-circuited.The adder 4 is then obviously eliminated and the output of switch 24connected directly to input 12 of the clamping circuit.

The complex circuits of FIG. 2 and their modifications have theadvantage of perfectly fixing the frequencies F and F',,. This result isnot obtained so perfectly, if a constant D-C component is simply addedto signal A only one reference wave at frequency F being then used inthe circuit of FIG. 2. Besides this would not be satis' factory for thephase synchronizing.

The two methods may however be advantageously combined as follows: thecircuits 101 and 102 normally comprise clamping circuits, for clampingsignals A and A before they are clipped, according to known art, whensignals A and A are transmitted according to the same modulationstandards. It is thus possible to add to signal A during this clampingoperation, the desired constant D-C component by modifying the referencevoltage used for clamping signal A accordingly.

Under those circumstances, the feedback circuit using two referencesignals of FIG. 2, in so far as the resting frequencies are concerned,no longer has the task of compensating for a basic gap but only ofcompensating for inaccuracy errors.

It is also possible to preemphasize, clamp and clip signals A and A atthe output of switch 24. In this case the clamping is alternatelyeffected with two reference potentials, changing from one clampingperiod to the other; the reference voltage is supplied by the output ofa switch having two inputs. Here again the operation of the feedbackloop is made easier.

It will be seen that the transmitter which has been described allows theapplication of the above-mentioned system for reducing subcarriervisibility.

This system is based upon a constant initial phase of the modulatedsubcarrier supplied by the frequency modulator at the start of each lineactive duration (corresponding to the transmission of picture signals)and subsequent phase-shifts of 180 degrees, each of which affects awhole number of line periods.

Those phase-shifts have only the object of producing opticalcompensations between the picture lines corresponding to thetransmission of the same chrominance signal, either A or A Under theseconditions it is indifferent if prior to the phase reversals, thesubcarrier has not the same initial phase at the start of the activeline periods corresponding to the transmission of A as at the start ofthe active line periods corresponding to the transmission of A as longas this phase is constant in one case as in the other, which result isobtained with the circuit of FIG. 2, and its variations.

This initial phase may further be made the same for the transmission ofboth signals (A and A through slightly delaying, by means of a delaydevice, the instant at which pulses J are applied to one of the harmonicgenerators relative to the instant to which they are applied to theother. Experience shows that the synchronizing is facilitated.

FIG. 4 shows the modifications to be applied to the subcarrier channelof receivers, in the example described.

The modulated subcarrier is applied at 60 to the decoding filter 61whose gain vs. frequency curve compensates for that of the coding filter(FIG. 1). The output of the filter 61 feeds in parallel a direct channel62 and a delay channel 63, which delays the signals passing therethroughby one line period relative to the signals passing through the directchannel.

The outputs of the direct and delay channels are connected to the twosignal inputs 65 and 66 of a switch 64 controlled by means of signalsapplied to its control inputs and 91, and directing the subcarrier(direct or delayed) respectively to its output 67 when it is modulatedby the signal A and to its output 68 when it is modulated by the signalA The output 67 feeds a frequency demodulator 69 consisting of afrequency discriminator preceded by an amplitude limiter, and the output68 a frequency demodulator 70, having the same components.

The frequency discriminator of the demodulator 69 is centred on thefrequency F and the discriminator of the demodulator 70 on the frequencyF' Moreover the demodulator 69 is preferably so designed that itsupplies the signal A with a negative coefficient, while the demodulator70 supplies the signal A with a positive coefiicient. The demodulationslopes of the demodulators determined for example, by the thresholds ofthe preceding limiters, are chosen taking into account the modulationstandards for each of the two signals, and the precise signals,proportional to A and A respectively, which it is preferred to obtain atthe outputs of the demodulators.

In particular, the slopes of the demodulators may be chosen to supplythe signals R-Y and B-Y directly, instead of A and A The output signalsof the demodulators are finally deemphasized in the deemphasis filters71 and 72, connected to the outputs of the demodulators 69 and 70respectively.

During the checking periods, the outputs of each dcrnodulator, 69 and70, and of each deemphasis filter, 71 and 72, respectively provide asignal, which differs according to whether the phase of switch 64 iscorrect or incorrect. Consequently these signals can be used as in knownart, in the switch control circuit. It can be easily seen that it isadvantageous to take a positive and a negative for the example describedabove.

These identification signals present the twofold advantage (a) of theirlong duration (12) of their high magnitude relatively to the coloursignals since it is possible to 13 encroach for their transmission, overthe excess of the total frequency swing relatively to the restrictedfrequency swing.

Of course, the invention is not limited to the embodiments shown. Inparticular, it is possible, although less advantageously, to use twomodulated oscillators respectively for the transmission of A and A Whatis claimed is:

l. A colour television system comprising a transmitter and at least onereceiver, in which the transmitted composite video signal comprises aluminance signal and a colour subcarrier, said transmitter including acolour channel comprising means for generating two colour signals A andA means for preemphasizing said two colour signals, means for fixing theresting frequency of said subcarrier to a value F and to a value F'means for alternately frequency modulating at line frequency saidsubcarrier resting at frequency F by said pre-ernphasized signal A andsaid subcarrier resting at frequency F by said pre-emphasized signal A acoding filter whose gain increases on either side of a frequency Fwithin the variation range of the instantaneous frequency of thefrequencymodulated subcarrier, said coding filter being fed by thefrequency modulated subcarrier before its addition to the luminancesignal, said frequencies F and F differing from said frequency F saidreceiver including a colour channel fed by said frequency-modulatedsubcarrier comprising a decoding filter which compensates for thefrequency selective action of the coding filter, a direct channel and adelay channel fed in parallel by the output of said decoding filter,said delay channel delaying the signal propagating therethrough by oneline period, switching means having a first and a second output andWhose inputs are respectively fed by said direct and delay channels,said switching means directing the subcarrier to said first and to saidsecond output according to whether it is modulated by A or A and a firstand a second frequency discriminator respectively centered on saidfrequencies F and F and respectively coupled to said first and secondoutput of said switching means.

2. A colour television system as claimed in claim 1, wherein said coloursignals A and A respectively being of the form A (RY)/K and A (BY) /Kwhere K is a negative constant, and K a positive constant, wherein F issmaller than F and both being respectively located on either side of F3. A colour television as in claim 2, wherein said means for fixing theresting frequency comprises a single oscillator having a modulationinput and a voltage-controlled frequency characteristic, two voltagesdiffering by a value corresponding to the difference F F' and 14 meansfor applying said two voltages at the same time as signals A and A 4. Acolour television system as claimed in claim 3, where F and F aredifferent entire multiples of the line frequency, and where saidtransmitter comprises a feedback loop with a phase comparator forsynchronizing the frequency of said modulated oscillator alternately tothe frequency of a reference wave with the frequency F and to thefrequency of a reference wave with the frequency F during the back porchof the line blanking interval.

5. A colour television receiver for receiving a composite video signalcomprising a luminance signal and a colour subcarrier situated withinthe frequency band of said luminance signal, said colour subcarrierbeing alternately frequency-modulated by two pre-emphasized chrominancesignals A and A the alternation of A and A occurring at the linefrequency, said colour subcarrier taking on two resting frequencies Fand F for modulation by A and A respectively, said resting frequencies Fand F being situated on either side of a frequency F representing themaximum attenuation of a coding filter to which the modulated subcarrieris submitted before being added to said luminance signal, said codingfilter constituting a part of a noise protection device for the coloursubcarrier, said receiver comprising a colour channel fed by saidsubcarrier, said colour channel comprising a decoding filter whichcompensates for the frequency selective action of the coding filter, adirect channel and a delay channel fed in parallel by the output of saiddecoding filter, said delay channel delaying the signal propagatingtherethrough by one line period, switching means whose inputs arerespectively fed by said direct and delay channels and having a firstand a second output, said switching means directing the subcarrier tosaid first and said second output according to whether it is modulatedby A or by A and a first and a second frequency discriminatorrespectively centered on frequencies F and F and respectively coupled tosaid first and second output of said switching means.

References Cited FOREIGN PATENTS 1,338,876 8/1963 France. 1,370,1417/1964 France.

RICHARD MURRAY, Primary Examiner J. MARTIN, Assistant Examiner U.S. Cl.X.R. 178-5 .4

